1. Field of the Invention
The present invention refers to a method for detecting the position of a rotor of a DC motor and to the related circuit.
2. Description of the Related Art
A DC motor comprises a permanent magnet that constitutes the rotor and a stator having a plurality of windings. This type of motor is commonly used to operate computer hard disks, DVDs (Digital Video Disk), CDs etc.
In most cases, DC motors have three phases and the driving circuit generally comprises integrated circuits whose output stages, typically half-bridges, drive the windings cyclically.
For the starting of a DC motor it is necessary to supply the windings in the correct combination of phases, and in order to do so it is necessary to know the position of the rotor.
The detection of the position of the rotor is based normally on the fact that the winding impedance depends on the position of the rotor. For this reason a small current is provided sequentially in each of the combinations of the windings and the voltage across an external resistance placed in series with each of the windings combinations is measured.
Particularly for a motor with three phases, U, V, and W, a current is made flowing sequentially between the following couples of phases: U-V, U-W, V-W, V-U, W-U, W-V, and the time, for each couple of phases, that the voltage across the resistance takes to arrive at a reference value is measured. Or, alternatively, the voltage across the resistance is measured after a prefixed time.
The value of the resistance varies normally from 100 milliohms to 500 milliohms and cannot be integrated inside of the integrated circuits and therefore it is placed outside with consequent increasing of the number of pins of the integrated circuit, bulk on the printed circuit board and increasing of the associated cost. Besides, the resistance causes a limit on the maximum voltage that can be furnished to the motor and has a not negligible power dissipation. In particular, such drawbacks take a remarkable importance in mobile applications and for devices supplied at low voltage (5 V and 3,3 V).